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16
industrial communications handbook 2016
Outdoor, or at least a relatively large distance, with
not much inbetween, the Friis link equation, given
above in
Equation 3.4
works best. But it depends on
line-of-sight. Now this is not as simple as it may at first
seem. At least 60% of the first Fresnel zone must be clear
of any obstruction, otherwise the link will be intermit-
tent at best.
Remember that the Fresnel zone is a three dimen-
sional ellipsoid between the transmitter and the receiv-
er. The earth gets in the way, trees, hills, buildings …
Assuming a symmetrical link, with the worst obstruc-
tion in the middle of the link (e.g. earth), we can get the
First Fresnel zone radius in metres from
Equation 3.5
.
F
D
f
1 m
km
8.656
( )
( )
(
)
= ×
GHz
(3.5)
Or, since we are only really interested in 2,45 and
5,8 GHz, this reduces to:
and
The first Fresnel zone under these conditions is shown
in
Table 3.1
.
Table 3.1: Fresnel zone radii in metres at the centre of
a symmetric ellipsoid.
height at the ends of the link may need adjustment. A
fully redundant system has several antennas, positioned
vertically to account for this. An example is the classic
Microwave towers seen dotted around the country for
telephony in the days before fibre.
Finally, remember that 2,45 GHz is the frequency
used in your microwave oven to cook your potato, de-
signed to make water molecules within it vibrate and
cause heating. Since WiFi is at the same frequency, and
a tree has leaves containing water, a highly directional
link through a tree works in winter in the absence of
leaves, but is useless in summer with luxuriant growth.
It gets worse in the rain when all the leaves are nice and
wet.
Rain, in itself, at these frequencies is a problem,
hence the clamoring for TV frequencies that will be
freed up after digital migration, as VHF does not suffer
from such efficient absorption.
When planning an outdoor link, a link margin is cru-
cial to the success of the link, and strongly influences
cost. Hence, how much do you need to spend to get a
reliable link? How long is a piece a of string?
A margin of ‘only’ 10 dB means 10 times the power.
3.3 Indoor and diversity
The majority of industrial communications will occur in
an environment that would be classified as indoor. This
is defined by lots of clutter, both metallic and nonmetal-
lic. Metallic clutter introduces a fully reflective surface,
and strongly reflects the electromagnetic wave, interact-
ing with the strong forward signal, leading to interfer-
ence: Constructive and Destructive!
Remember from before that a quarter wavelength,
90
°
, exists between a point of absolute destruction and
beautiful addition. Thus, at 2,45 GHz WiFi, that is 30 mm.
On this basis, it can be seen that communication at
this frequency in a busy environment is simply impos-
sible.
The only way WiFi actually works is by having
Di-
versity
.
Diversity ensures that when one antenna is in a de-
structive interference zone, there is another antenna
that can still receive. This of course requires two differ-
ent radios, and the ability to be able to switch between
the signals very rapidly indeed: requiring a computing
platform to decide which signal is stronger. Even the
Link distance [km] F1 [m] 2,45 GHz F1 [m] 5,8 GHz
1
5,5
3,2
2
7,8
4,5
3
9,6
5,5
4
11,1
6,3
5
12,4
7,0
Table 3.1
thus also shows why the higher frequency is
most often used for longer distance links, as no one
wants a 13 m mast at both ends of a 5 km link! (There
are other considerations, but this is a positive!)
A point-to-point link that works perfectly in Winter
may not work when Summer comes around. Over a ki-
lometre or so link, the refractive index of the air chang-
es, especially with temperature, changing the so-called
‘K-factor’ which accounts for earth curvature. So the